Detector for Use in Charged-Particle Microscopy

ABSTRACT

A method of investigating a sample using a charged-particle microscope is disclosed. By directing an imaging beam of charged particles at a sample, a resulting flux of output radiation is detected from the sample. At least a portion of the output radiation is examined using a detector, the detector comprising a Solid State Photo-Multiplier. The Solid State Photo-Multiplier is biased so that its gain is matched to the magnitude of output radiation flux.

This Application claims priority from U.S. Provisional Application61/442,546, filed Feb. 14, 2011, which is hereby incorporated byreference.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method of investigating a sample using acharged-particle microscope.

BACKGROUND OF THE INVENTION

Electron microscopy is a well-known technique for imaging microscopicobjects. The basic genus of electron microscope has undergone evolutioninto a number of well-known apparatus species, such as the TransmissionElectron Microscope (TEM), Scanning Electron Microscope (SEM), andScanning Transmission Electron Microscope (STEM), and also into varioussub-species, such as so-called “dual-beam” tools (that additionallyemploy a “machining” beam of ions, allowing supportive activities suchas ion-beam milling or ion-beam-induced deposition, for example). Intraditional electron microscopes, the imaging beam is “on” for anextended period of time during a given imaging session; however,electron microscopes are also available in which imaging occurs on thebasis of a relatively short “flash” or “burst” of electrons, such anapproach being of potential benefit when attempting to image movingsamples or radiation-sensitive specimens, for example.

In current electron microscopes [and other charged-particlemicroscopes], use is often made of a detector that employs an evacuatedphoto-multiplier tube (PMT) in conjunction with a scintillator. In sucha set-up, output electrons emanating from the sample move toward andstrike the scintillator (which will often be maintained at anaccelerating potential of the order of a few kV with respect to thesample), thus causing the production of photonic radiation (i.e.electromagnetic radiation, such as visible light) that, in turn, isdirected (e.g. with the aid of a light guide) to a photo-emissivecathode of the PMT, from which it triggers the ejection of one or morephotoelectrons. Each such photoelectron traverses a series ofhigh-voltage dynodes—each of which emits a plurality of electrons foreach impinging electron (cascade effect)—so that a greatly augmentednumber of electrons eventually leaves the last dynode and strikes adetection anode, producing a measurable electric current or pulse. Thecathode, dynodes and anode are all located in an evacuated vitreoustube.

This known detector set-up (often referred to as an Everhart-Thornleydetector) has certain drawbacks. For example, the vitreous tube of thePMT is necessarily quite bulky, seeing as it has to accommodate multipleelectrodes in a specific mutual configuration, and has to support highinternal vacuum. Such bulkiness is exacerbated by the fact that eachelectrode requires an electrical connection through the wall of thevitreous tube to the tube's exterior, where it is connected via anelectrical cable to a high-voltage source (typically operating in the kVrange). In addition, a light guide between the scintillator and the PMTmay necessarily be quite long (e.g. due to spatial restrictions inplacement of the PMT), and this will generally lead to some degree ofsignal loss. Moreover, the very principle of operation of the PMTresults in a relatively large ultimate electrical current for eachelectron that strikes the scintillator; consequently, in a scenario inwhich irradiation of a sample by an imaging beam produces a relativelylarge flux of output radiation from the sample, this can result in anexcessive electrical current at the anode of the PMT. To mitigate thiseffect, one can attempt to attenuate the input to the PMT in some way,e.g. by making the employed scintillator less sensitive, but such actionwill generally tend to complicate the detector set-up even further.

Accordingly, there is a need to provide a radically alternativedetection scenario to that set forth above.

SUMMARY OF THE INVENTION

The invention relates to a method of investigating a sample using acharged-particle microscope. By directing an imaging beam of chargedparticles to a sample, a flux of output radiation emanates from thesample. The output radiation is detected and examined using a SolidState Photo-Multiplier.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter. It should be appreciated by those skilled in the art thatthe conception and specific embodiments disclosed may be readilyutilized as a basis for modifying or designing other structures forcarrying out the same purposes of the present invention. It should alsobe realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the inventionas set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a cross-sectional view of a particular embodiment of acomposite Multi-Pixel Photon Counter-based detector that, in accordancewith the present invention, can be used in a charged-particlemicroscope.

FIG. 2 shows a graph of gain versus bias for a particular Multi-PixelPhoton Counter, obtained using the insights underlying the currentinvention and exploitable in a detector for use in a charged-particlemicroscope according to the current invention.

FIG. 3 shows a longitudinal cross-sectional view of a particularembodiment of a charged-particle microscope—in this specific case aTEM—according to the current invention.

FIG. 4 shows a longitudinal cross-sectional view of a particularembodiment of another charged-particle microscope—in this specific casea SEM—according to the current invention.

FIG. 5 shows a longitudinal cross-sectional view of a particularembodiment of yet another charged-particle microscope—in this specificcase a FIB tool—according to the current invention.

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In research leading to the invention, the inventors realized that thebulkiness of detectors based on evacuated PMTs was a bottleneck to theongoing desire to develop more compact and/or innovative types ofelectron microscope [and other types of charged-particle microscope].The inventors therefore embarked upon a systematic engineering projectto develop a totally new type of detector for use in electronmicroscopy, with the aim of overcoming the seemingly unassailable hurdleimposed by said bottleneck.

To start off with, the inventors turned their attention to photodiodes(PDs), which have the advantages of being compact and of requiring onlya relatively low operating voltage, and the additional merit of beingcomparatively inexpensive. However, a drawback of PDs is that they donot have a significant/satisfactory amplification effect, making themrelatively unsuitable for detection of weak signals. To overcome thisdrawback, the inventors considered using so-called avalanche photodiodes(APDs), which employ an electrical bias to produce an “avalanche” ofelectrons for a given triggering photon. However, although these devicesdid produce an amplification effect, it was considered to be too weakfor many applications in electron microscopy.

Next, the inventors performed experiments with Geiger-APDs (or GAPDs),which are APDs operated in so-called Geiger mode, whereby the employedoperating bias is greater than the breakdown voltage of the diodeconcerned, causing a breakdown shower of electrons (self-sustaineddischarge) for a given triggering photon. However, just like a catapultthat needs to be rewound after firing, a GAPD needs to “quench” and“recharge” after each triggering event, and this process requires a spanof time that is referred to as the “recovery time” or “dead time” of thedevice, and is typically in the nanosecond range. If a photon impingeson the device during this dead time, the device cannot produce thedesired discharge in response thereto, as a result of which the photonin question will go undetected. Consequently, such a set-up would notlend itself to the detection of relatively high fluxes of photons.

The inventors next postulated that the disadvantageous effect of theabove-mentioned dead time could be circumvented if one were to use anarray of detection diodes, the associated argumentation being that, atany given time, some of the diodes in such an array would beexperiencing their dead time, but others would be trigger-ready;consequently, in general, any photon arriving at the array would alwaysbe met by at least some diodes that were ready and able to trigger, sothat such a photon would have an increased probability of beingdetected. The inventors realized that, in order to offer optimumplacement versatility, such an array would need to be relativelycompact, and preferably in the form of an integrated (i.e. on-chip)device. However, prior to taking steps to contact a semiconductorfoundry to discuss the design and manufacture of such an integrateddevice, they discovered by coincidence that, in the field of high-energyparticle physics experimentation (such as conducted at CERN, FermiLab,etc.), researchers hunted elusive and exotic sub-atomic particles usingvery sensitive pulse-counting devices called Multi-Pixel Photon Counters(also known by names such as Solid State Photo-Multipliers (SSPMs),Silicon Photo-Multiplier (SiPMs), on-chip pixelated APD arrays, etc.),which were essentially on-chip arrays of the order of about 10³-10 ⁴APDs with shared/common detection circuitry, and turned out to becommercially available, e.g. from the firm Hamamatsu in Japan under thename MPPC®. It is noted that also Multi-Pixel Photon Counters withbetween 10 and 1000 APD's are available.

It was felt ab initio that, in view of the very disparate needs ofhigh-energy particle physics researchers as compared to those ofelectron microscopy engineers, such Multi-Pixel Photon Counters wouldnot offer satisfactory performance in an electron microscope; however,it was considered to be a fruitful exercise to at least undertake toperform some investigative experiments with them.

As expected, preliminary experiments suggested that the Multi-PixelPhoton Counters were unsuitable for use as detectors in electronmicroscopy. Their very name—“photon counters”—indicates that they areintended to deal with very low detection fluxes, and they were indeedfound to be severely saturated at even moderate detection fluxes, suchas commonly occur in electron microscopy.

However, in a chance event, the inventors discovered that, when a testMulti-Pixel Photon Counter was operated at bias levels below thespecifications stipulated by the manufacturer, it demonstrated a muchlower gain. This phenomenon caught the interest of the inventors, andthey embarked upon a full series of experiments to investigate itfurther. Eventually, they were surprised to observe that, when aMulti-Pixel Photon Counter was operated within a relatively narrowvoltage band outside of spec, its gain varied in a reproduciblemanner—without saturation effects—through several orders of magnitude,according to a weak “S-shaped” response curve; for example, for aspecific MPPC specimen having a specified operating voltage above 74 V,it was found that the gain varied smoothly through four orders ofmagnitude as a function of applied bias in the range ˜69-73 V.

The inventors seized upon this realization and, according to the presentinvention, conceived a successful implementation of a Multi-Pixel PhotonCounter as a detector for electron microscopy [and, ultimately, othertypes of charged-particle microscopy]. According to this implementation,the operating bias of the Multi-Pixel Photon Counter (i.e. on-chippixelated Geiger-mode Avalanche Photodiode array, Solid StatePhotomultiplier (SSPM), etc.) is carefully adjusted so as to endow itwith a gain value that is matched to a particular detectioncircumstance; for example, for situations in which a low flux of outputradiation is expected to emanate from a sample, a relatively large biascan be applied to the Multi-Pixel Photon Counter (thus giving itrelatively high gain) whereas, under circumstances in which a higherflux of output radiation is expected to emanate from the sample, thegain of the Multi-Pixel Photon Counter can be appropriately “choked” byoperating it at a relatively low bias. The exact bias to be applied tothe Multi-Pixel Photon Counter so as to realize a desired gain can bedetermined on the basis of a bias/gain calibration curve drawn up priorto using the Multi-Pixel Photon Counter in an electron microscope [orother charged-particle microscope].

Despite the fact that a Multi-Pixel Photon Counter used in this manneris operating outside the specifications stipulated by its manufacturer,the inventors have observed that it nevertheless generally demonstratesacceptable temperature stability, signal-to-noise ratio (SNR) andreproducibility; although the SNR tends to be significantly lower (thenoise significantly larger) than typically achieved from an evacuatedPMT or from a Multi-Pixel Photon Counter operating withinspecifications, it does not tend to impede measurement accuracy to anygreat extent.

The novel detector developed by the inventors in accordance with thepresent invention is much smaller than an evacuated PMT (or otherconventional detector types used in charged-particle microscopy), andoperates at a much lower voltage. Accordingly, it can be placed inlocations and used in circumstances that are not possible in the case of(inter alia) evacuated PMT-based detectors, thus opening the door to awhole scala of new types of charged-particle microscope. For example:

-   -   In a charged-particle microscope according to the present        invention, the novel detector can be located in very close        proximity to the sample being investigated, since the detector        is much more compact than an evacuated PMT. An advantage of such        a configuration is that it generally allows the detector to more        efficiently capture output radiation emanating from the sample.        Specifically, a Multi-Pixel Photon Counter can be placed very        close to an associated scintillator (located adjacent to a        sample), removing the need for a relatively long light guide        between the two, thus helping to curtail signal loss.    -   The detector according to the current invention can be wholly        located within the particle-optical column (objective lens) of a        charged-particle microscope. Such a configuration has hitherto        been difficult in the case of evacuated PMT-based detection,        predominantly because of the bulk associated with the evacuated        PMT, but also because of the electric fields associated        therewith. An “in-lens” detector configuration has the advantage        of allowing greater freedom of choice as regards the so-called        “working distance” of the microscope; in particular, one can        realize a shorter working distance and, accordingly, reduce the        effect of lens aberrations (which scale with working distance).    -   Attendant to the previous example, one can comprise the current        invention in a so-called “immersion lens”, i.e. a set-up in        which the sample resides in an electric or magnetic field of the        lens. Since output electrons emanating from such a sample cannot        escape from the lens to an external detector, the detector will        instead have to be located within the lens, and will have to be        capable of operating satisfactorily in an electric/magnetic        field. The detector arrangement according to the current        invention is compatible with these requirements.    -   As set forth above, evacuated PMTs tend to “overload” when used        to measure high fluxes of output radiation emanating from a        sample. However, the detector according to the present        invention, which has its gain tuned according to the expected        detection flux, does not suffer from this problem.    -   The invention lends itself to application in charged-particle        microscopy in which so-called cathodoluminescence (CL) is        measured. Because the detector according to the invention is so        compact and, accordingly, can be placed close to the sample, it        affords a larger angular aperture to capture CL photons.

In a given embodiment of a charged-particle microscope according to theinvention, the detector additionally comprises a scintillator. In such ascenario, the Multi-Pixel Photon Counter discussed above acts as ameasuring element and the scintillator acts as a converting element,serving to convert output electrons emanating from the sample intophotons that then impinge upon the measuring element.

In an alternative scenario (e.g. when measuring CL radiation), theMulti-Pixel Photon Counter discussed above is used to directly detectoutput photons emanating from the sample, which can be done without theintermediary of a converting scintillator.

Embodiment 1

Multi-Pixel Photon Counters are commercially available from firms suchas Hamamatsu Photonics KK, Japan (for example). A Multi-Pixel PhotonCounter typically comprises a 2-dimensional array of several hundred orseveral thousand individual Geiger-APDs, integrated on a small chip.Such a chip typically has lateral dimensions of the order of about 3×3mm². In some cases, such chips can be housed in a (metal, ceramic orplastic) canister provided with electrical connection leads; however,such a canister is not necessary, and “naked” Multi-Pixel Photon Counterchips are also commercially available.

To appreciate the scale of such Multi-Pixel Photon Counter chips, thefollowing comparison is merited. As a reference, an evacuated PMT(photomultiplier tube) with a (typical) diameter of the order of about2½ cm and a (typical) length of the order of about 10 cm will have avolume of the order of about 80 cm³. In contrast, a canister as referredto above will typically have a volume of the order of about 1 cm³ orless, making it almost a hundred times smaller than said evacuated PMT.On the other hand, a “naked” Multi-Pixel Photon Counter chip mounted ona thin substrate (such as a sheet of glass, for example) will typicallyhave a volume of the order of about 3×3×1 mm³=9 mm³=0.009 cm³, making itapproximately ten thousand times smaller than an evacuated PMT.

This scale difference allows a very significant reduction in detectorsize as compared to evacuated PMTs (or other types of detector, such asconventional solid state detectors, for example), thus allowingMulti-Pixel Photon Counter -based detectors to be located in confinedspaces that are too cramped for an evacuated PMT (or other knowndetector types). Moreover, because Multi-Pixel Photon Counters are sosmall, and also very much cheaper than prior-art detectors, it becomespossible to employ several of them in unison—which allows more versatiledetection possibilities as compared to the use of a single, bulkydetector; for example, one could surround a sample by a whole “cloud” ofsuch Multi-Pixel Photon Counter detectors, allowing output radiationemanating from the sample to not only be detected, but also to beangularly/directionally resolved.

It is worth mentioning that such a multitude of detectors can be formedfrom completely separated chips, but can also be integrated on one die,preferably with a hole in the middle for passing the beam of chargedparticles to the sample.

In addition to these merits as regards size, cost and novel measurementconfigurations, a detector according to the invention has a furtheradvantage: because a Multi-Pixel Photon Counter is comprised of APDs,its operation is not impeded to any significant extent by magneticfields in its vicinity. On the other hand, since the operation of (forexample) an evacuated PMT relies on the use of acceleratingelectrostatic fields generated between electrode pairs, its functioningcan be detrimentally affected by environmental magnetic fields ofsignificant magnitude. Because a Multi-Pixel Photon Counter isrelatively insensitive to electric/magnetic fields, it can besuccessfully deployed in locations that are precluded to an evacuatedPMT; for example, it can be located within a particle-optical lens.

The inventors have surprisingly shown that a Multi-Pixel Photon Counter,when operated in a particular manner outside the manufacturer'sspecifications, can be successfully used to measure an incomingradiative flux across about 5 decades of magnitude, without beingimpeded by saturation effects, and with satisfactory reproducibility andsignal-to-noise ratio. To this end, the Multi-Pixel Photon Counter isconnected to a power supply providing an adjustable electrical bias, andthis bias is varied so as to adjust a gain value of the Multi-PixelPhoton Counter. By appropriately choosing the employed bias value, onecan match the device gain to the (expected or observed) magnitude of theincoming radiative flux to be measured, creating a scenario whereby theMulti-Pixel Photon Counter consistently operates below a saturationthreshold for the device (i.e. up to an acceptable saturation level).

In general, the thermal sensitivity of a Multi-Pixel Photon Counteroperated in this manner was found by the inventors to be acceptable.However, to the extent that, in a particular application, the thermalsensitivity is considered to be a more critical issue, one can alwaysresort to one or both of the following steps:

-   -   Prevention, whereby one endeavors to keep the temperature of the        Multi-Pixel Photon Counter as stable as possible;    -   Correction, whereby the temperature of/at the Multi-Pixel Photon        Counter is continually monitored, and any changes therein are        compensated therefor by appropriate (slight) adjustments to the        applied bias value.

In respect of this novel application of a Multi-Pixel Photon Counter asa detector in a charged-particle microscope, the following should benoted:

-   -   A Multi-Pixel Photon Counter can be used according to the        invention to directly measure incoming photonic radiation, such        as CL radiation; such measurement does not require the        intermediary of a converting element such as a scintillator.    -   A Multi-Pixel Photon Counter can also be used in accordance with        the invention to indirectly measure incoming particulate        radiation, by employing a scintillator to convert a flux of        particles (such as secondary or backscattered electrons, or        ions) into photons, which then impinge upon the Multi-Pixel        Photon Counter.

The next Embodiment will be devoted to a particular set-up that lendsitself to such indirect measurement.

Embodiment 2

FIG. 1 shows a cross-sectional view of a particular embodiment of acomposite Multi-Pixel Photon Counter-based detector that, in accordancewith the present invention, can be used in a charged-particlemicroscope. The figure shows a (naked) Multi-Pixel Photon Counter chip 1that is separated from a scintillator 3 via an interposed layer 5 ofoptically transparent material, such as glass or a suitable type ofgrease, for instance. The scintillator 3 may comprise a YAG (YttriumAluminium Garnet) crystal, for example.

The interposed (light guide) layer 5 serves to (partially) match therefractive indices of the scintillator 3 and Multi-Pixel Photon Counter1, and also to electrically isolate them from one another. In use, theMulti-Pixel Photon Counter 1 will be operated at a relatively lowvoltage, whereas the scintillator 3 will often be maintained at arelatively high electrical potential (typically of the order of kV). Toprevent arc-over, the sandwiched assembly 3, 5, 1 is partiallyencapsulated in a molded insulating jacket 7, which may comprise a(vacuum-compatible) substance such as silicon rubber or an epoxy resin,for example. This jacket 7 has a form/shape that leaves a substantialportion of a face of the scintillator 3 exposed to incoming particleradiation. If desired, this exposed face of the scintillator 3 (remotefrom the Multi-Pixel Photon Counter 1) may be thinly metallized, so asto reflect photons generated in the scintillator 3 towards theMulti-Pixel Photon Counter 1. Such a metallization is often also desiredto form a conductive surface on the (often insulating) scintillator, asotherwise charging could occur.

Also shown in the figure is a signal cable 9, which carries the outputsignal from the Multi-Pixel Photon Counter 1 to a controller (notdepicted).

Embodiment 3

FIG. 2 shows a graph of gain versus bias for a particular embodiment ofan Multi-Pixel Photon Counter, obtained using the insights underlyingthe current invention and exploitable according to the invention in adetector for application in a charged-particle microscope. In thisspecific case, the Multi-Pixel Photon Counter is a type/modelS10931-25P, obtainable from Hamamatsu Photonics KK, Japan, andcontaining a 2-dimensional array of 14400 Geiger-APDs in a 3×3 mm² die(chip) area. For this particular Multi-Pixel Photon Counter, themanufacturer specified an operating voltage of approximately 74V;however, for other Multi-Pixel Photon Counters, manufacturers canspecify different operating voltages (depending inter alia on thedetails of the integrated circuit design in the MPPC in question).

In a test associated with the present invention, the Multi-Pixel PhotonCounter in question was subjected to an out-of-spec operating voltage.The graph in FIG. 2 shows the surprising results, whereby, when theMulti-Pixel Photon Counter is operated within a relatively narrowsub-spec voltage band, its gain is observed to vary in a reproduciblemanner—without saturation effects—through several orders of magnitude,according to a weak “S-shaped” response curve.

The inventors think that the observed behavior of the Multi-Pixel PhotonCounter may be explained as follows.

-   -   At different positions in the bias/gain curve, the mechanism by        which the Multi-Pixel Photon Counter creates gain changes.    -   At bias values ≧74V, the device is in nominal photon counting        mode, as developed by the manufacturer.    -   Below that, e.g. in the range ˜74-70V, avalanche firing        probability decreases, an avalanche has lower gain, and recovery        time is shorter; consequently, the device can handle a higher        incoming radiation rate without saturation, at a lower gain.    -   At yet lower bias values, e.g. ˜70-40V, the device no longer        operates in Geiger mode, but instead shows normal APD        characteristics: no saturation effect, internal gain ˜10-100        (electrons arising from secondary generation, but not from        breakdown effects).    -   At even lower bias values, e.g. ˜40-0V, there is no internal        gain. The device acts as normal photodiode, whereby one photon        generates one electron-hole pair.

In the illustrated graph, a spline fit has been made to the depicteddata points. This fit can subsequently be used to predict what biasvalue to use in order to optimally set the gain of the Multi-PixelPhoton Counter to a given value, allowing it to deal with a measured orexpected flux of incoming radiation without exceeding an acceptablesaturation level. If such an incoming flux is expected to be very weak,the Multi-Pixel Photon Counter can be used at (or close to) themanufacturer-stipulated operating voltage, so that it behaves as a pulsecounter; on the other hand, if an incoming flux is expected to be verystrong, the Multi-Pixel Photon Counter can be operated at a very lowbias, taking it out of Geiger mode and into APD or PD mode, if required(see previous paragraph).

Embodiment 4

FIG. 3 renders a longitudinal cross-sectional view of a particularembodiment of a TEM according to the current invention.

The depicted TEM comprises a vacuum housing 120 that is evacuated viatube 121 connected to a vacuum pump 122. A particle source in the formof an electron gun 101 produces a beam of electrons along aparticle-optical axis (imaging axis) 100. The electron source 101 can,for example, be a field emitter gun, a Schottky emitter, or a thermionicelectron emitter. The electrons produced by the source 101 areaccelerated to an adjustable energy of typically 80-300 keV (althoughTEMs using electrons with an adjustable energy of 50-500 keV, forexample, are also known). The accelerated electron beam then passesthrough a beam limiting aperture/diaphragm 103 provided in a platinumsheet. To align the electron beam properly to the aperture 103, the beamcan be shifted and tilted with the aid of deflectors 102, so that thecentral part of the beam passes through the aperture 103 along axis 100.Focusing of the beam is achieved using magnetic lenses 104 of thecondenser system, together with (part of the) objective lens 105.Deflectors (not depicted) are used to centre the beam on a region ofinterest on a sample 111, and/or to scan the beam over the surface ofthe sample.

The sample 111 is held by a sample holder 112 in such a manner that itcan be positioned in the object plane of objective lens(particle-optical column) 105. The sample holder 112 may be aconventional type of sample holder for holding a static sample in acontainment plane; alternatively, the sample holder 112 can be of aspecial type that accommodates a moving sample in a flow plane/channelthat can contain a stream of liquid water or other solution, forexample.

The sample 111 is imaged by a projection system comprising lenses 106onto fluorescent screen 107, and can be viewed through a window 108. Theenlarged image formed on the screen typically has a magnification in therange 10³x-10⁶x, and may show details as small as 0.1 nm or less, forexample. The fluorescent screen 107 is connected to a hinge 109, and canbe retracted/folded away such that the image formed by the projectionsystem 106 impinges upon primary detector 151. It is noted that, in suchan instance, the projection system 106 may need to be re-focused so asto form the image on the primary detector 151 instead of on thefluorescent screen 107. It is further noted that the projection system106 will generally additionally form intermediate images at intermediateimage planes (not depicted).

The primary detector 151 may, for example, comprise a Charge CoupledDevice (CCD) for detecting impinging electrons. As an alternative toelectron detection, one can also use a CCD that detects light—such asthe light emitted by a Yttrium Aluminium Garnet (YAG) crystal (forexample) that is bonded to the CCD, or connected thereto by opticalfibres (for example). It is noted that such a scintillator may be asingle crystal, but may also consist of a screen with the scintillatorbonded in powdery form. In such an indirect detector, the YAG crystalemits a number of photons when an electron hits the crystal, and aportion of these photons is detected by the CCD camera; in directdetectors, electrons impinge on the semiconductor chip of the CCD andgenerate electron/hole pairs, thereby forming the charge to be detectedby the CCD chip.

The image formed on the fluorescent screen 107 and on the primarydetector 151 is generally aberrated due to distortion produced by thelenses 106. To correct such distortion, multipoles 152 are used, each ofwhich may be a magnetic multipole, an electrostatic multipole or acombination thereof. In the current case, three levels/sets ofmultipoles are shown; however, a smaller number may also suffice, or, inother cases, a larger number of multipoles may be necessary, in order tocorrect the distortions with greater accuracy.

It should be noted that FIG. 3 only shows a schematic rendition of atypical TEM, and that, in reality, a TEM will generally comprise manymore deflectors, apertures, etc. Also, TEMs having correctors forcorrecting the aberration of the objective lens 105 are known, saidcorrectors employing multipoles and round lenses.

Where the imaging beam impinges on the sample 111, secondary radiationis generated in the form of secondary electrons, visible (fluorescence)light, X-rays, etc. Detection and analysis of this secondary radiationcan provide useful information about the sample 111. However, as isevident from FIG. 3, the vicinity of the sample 111 is rather cluttered,making it difficult to place conventional detectors here. The currentinvention obviates this problem, because a detector in accordance withthe current invention is very small, and also relatively insensitive toelectric/magnetic fields. Accordingly, FIG. 3 shows a supplementarydetector 130, which is embodied as an integrated Multi-Pixel PhotonCounter, connected to a variable voltage source 132 and biased inaccordance with the invention so as to ensure that it operates at anacceptable (sub-threshold) saturation level.

As here depicted, the detector 130 is positioned at the side of thesample 111 distal from the gun 101. However, this is a matter of designchoice, and a detector 130 may alternatively be positioned at the sideof the sample 111 facing the gun 101. Furthermore, the small size ofsuch inventive detectors 130 allows a plurality of them to be placed inthe vicinity of the sample 111, if desired; in such a case, one could,for example, designate different detectors 130 to investigate differenttypes of secondary radiation, and/or one could angularly/directionallyresolve the detected secondary radiation. Yet another possibility wouldbe to locate one or more of the detectors 130 more directly above and/orbelow the sample 111, placing them within (or at least very close to)the confines of the objective lens 105 and/or the projection system 106,for example.

One or more detectors 130 according to the invention could also be usedto detect other types of radiation emanating from the sample 111, suchas backscatter electrons, for example. As set forth above, a detector130 according to the invention can be used to directly measure photonicradiation, or to indirectly measure particulate radiation (with the aidof an intermediary scintillator).

Embodiment 5

FIG. 4 renders a longitudinal cross-sectional view of a particularembodiment of a SEM according to the current invention.

In FIG. 4, a SEM 400 is equipped with an electron source 412 and a SEMcolumn (particle-optical column) 402. This SEM column 402 useselectromagnetic lenses 414, 416 to focus electrons onto a sample 410,and also employs a deflection unit 418, ultimately producing an electronbeam (imaging beam) 404. The SEM column 402 is mounted onto a vacuumchamber 406 that comprises a sample stage 408 for holding a sample 410and that is evacuated with the aid of vacuum pumps (not depicted). Thesample stage 408, or at least the sample 410, may be set to anelectrical potential with respect to ground, using voltage source 422.

The apparatus is further equipped with a detector 420, for detectingsecondary electrons that emanate from the sample 410 as a result of itsirradiation by the imaging beam 404. In prior-art SEMs, one oftenresorted to using a bulky Everhart-Thornley detector to fulfil thusrole. However, according to the present invention, the detector 420 canbe advantageously embodied as an appropriately biased Multi-Pixel PhotonCounter, or as a plurality of Multi-Pixel Photon Counters placed in adistributed arrangement around the sample 410. The inventive detector420 is small enough to be placed within the SEM column 402, and/or invery close proximity to the sample 410, if desired. A mini-scintillatorcan be used in conjunction with the/each Multi-Pixel Photon Counter, inorder to effect the conversion of incoming secondary electrons intophotonic radiation.

In addition to the detector 420, this particular set-up (optionally)comprises a sensor 430, which here takes the form of a plate providedwith a central aperture 432 through which imaging beam 404 can pass. Theapparatus further comprises a controller 424 for controlling inter aliathe deflection unit 418, the lenses 414, 416, the detector 420 and thesensor 430, and displaying obtained information on a display unit 426.

As a result of scanning the imaging beam 404 over the sample 410, outputradiation, such as secondary electrons and backscattered electrons,emanates from the sample 410. In the depicted set-up, secondaryelectrons are captured and registered by the detector 420, whereasbackscattered electrons are detected by sensor 430. As the emanatedoutput radiation is position-sensitive (due to said scanning motion),the obtained (detected/sensed) information is also position-dependent.The signals from the detector 420/sensor 430, either severally orjointly, are processed by the controller 424 and displayed. Suchprocessing may include combining, integrating, subtracting, falsecolouring, edge enhancing, and other processing known to the skilledartisan. In addition, automated recognition processes, such as used inparticle analysis, for example, may be included in such processing.

In an alternative arrangement, voltage source 422 may be used to applyan electrical potential to the sample 410 with respect to theparticle-optical column 402, whence secondary electrons will beaccelerated towards the sensor 430 with sufficient energy to be detectedby it; in such a scenario, detector 420 can be made redundant.Alternatively, by substituting one or more of the inventive detectors420 for the sensor 430, these detectors 420 can assume the role ofdetecting backscattered electrons, in which case the use of a dedicatedsensor 430 can be obviated.

If desired, one can realize a controlled environment (other than vacuum)at the sample 410. For example, one can create a pressure of severalmbar, as used in a so-called Environmental SEM (ESEM), and/or one candeliberately admit gases—such as etching or precursor gasses—to thevicinity of the sample 410. It should be noted that similarconsiderations apply to the case of a TEM, e.g. as set forth inEmbodiment 4 above, whereby a so-called ETEM (Environmental TEM) can berealized, if desired.

Embodiment 6

FIG. 5 renders a longitudinal cross-sectional view of a particularembodiment of a FIB microscope according to the current invention.

FIG. 5 shows a FIB tool 500, which comprise a vacuum chamber 502, an ionsource 512 for producing a beam of ions along an optical axis 514, and aFIB column (particle-optical column) 510. The FIB column includeselectromagnetic (e.g. electrostatic) lenses 516 a and 516 b, and adeflector 518, and it serves to produce a focused ion beam (imagingbeam) 508.

A workpiece (sample) 504 is placed on a workpiece holder (sample holder)506. The workpiece holder 506 is embodied to be able to position theworkpiece 504 with respect to the focused ion beam 508 produced by theFIB column 502.

The FIB apparatus 500 is further equipped with a Gas Injection System(GIS) 520. The GIS 520 comprises a capillary 522 though which a gas maybe directed to the workpiece 504, and a reservoir 524 containing the gas(or a precursor substance used to produce the gas). A valve 526 canregulate the amount of gas directed to the workpiece 504. Such a gas maybe used in depositing a (protective) layer on the workpiece 504, or toenhance a milling operation performed on the workpiece 504, for example.If desired, multiple GIS devices 520 may be employed, so as to supplymultiple gases according to choice/requirement.

The FIB tool 500 is further equipped with a detector 530, which, as hereembodied, is used to detect secondary radiation emanating from thesample 504 as a result of its irradiation by the ion beam 508. Thesignal from the detector 530 is fed to a controller 532. This controller532 is equipped with a computer memory for storing the data derived fromthis signal. The controller 532 also controls other parts of the FIB,such as the lenses 516 a/b, the deflector 518, the workpiece holder 506,the flow of the GIS 520 and the vacuum pumps (not depicted) serving toevacuate the chamber 502. In any case, the controller 532 is embodied toaccurately position the ion beam 508 on the workpiece 504; if desired,the controller 532 may form an image of detected/processed data onmonitor 524.

In analogy to the Embodiments above, the role of this detector 530 may,in accordance with the invention, be fulfilled by one or more suitablybiased Multi-Pixel Photon Counters, which may be deployed in conjunctionwith one or more mini-scintillators, depending on the nature of theradiation to be detected.

In the present disclosure, the invention will—by way of example—often beset forth in the specific context of electron microscopes. However, suchsimplification is intended solely for clarity/illustrative purposes, andshould not be interpreted as limiting.

As used throughout this text, the ensuing terms should be interpreted asfollows:

-   -   The term “charged particle” refers to an electron or ion        (generally a positive ion, such as a Gallium ion or Helium ion,        for example).    -   The term “microscope” refers to an apparatus that is used to        create a magnified image of an object, feature or component that        is generally too small to be seen in satisfactory detail with        the naked human eye. In addition to having an imaging        functionality, such an apparatus may also have a machining        functionality; for example, it may be used to locally modify a        sample by removing material therefrom (“milling” or “ablation”)        or adding material thereto (“deposition”). Said imaging        functionality and machining functionality may be provided by the        same type of charged particle, or may be provided by different        types of charged particle; for example, a Focused Ion Beam (FIB)        microscope may employ a (focused) ion beam for machining        purposes and an electron beam for imaging purposes (a so-called        “dual beam” microscope), or it may perform machining with a        relatively high-energy ion beam and perform imaging with a        relatively low-energy ion beam. On the basis of this        interpretation, tools such as the following should be regarded        as falling within the scope of the current invention: electron        microscopes, FIB apparatus, EBID and IBID apparatus        (EBID=Electron-Beam-Induced Deposition; IBID=Ion-Beam-Induced        Deposition), Critical Dimension (CD) measurement tools,        lithography tools, Small Dual Beams (SDB), etc.    -   The term “particle-optical column” refers to a collection of        electrostatic and/or magnetic lenses that can be used to        manipulate a charged-particle beam, serving to provide it with a        certain focus or deflection, for example, and/or to mitigate one        or more aberrations therein.    -   The term “output radiation” encompasses any radiation that        emanates from the sample as a result of its irradiation by the        imaging beam. Such output radiation may be particulate and/or        photonic in nature. Examples include secondary electrons,        backscattered electrons, X-rays, visible fluorescence light, and        combinations of these. The output radiation may simply be a        portion of the imaging beam that is transmitted through or        reflected from the sample, or it may be produced by effects such        as scattering or ionization, for example.    -   The term “detector” refers to at least one detector somewhere in        the charged-particle microscope. There may be several such        detectors, of different types and/or in different locations. The        invention aims to embody at least one such detector according to        a specific form/functionality.    -   The term “electromagnetic” should be interpreted as encompassing        various manifestations of electromagnetism. For example, an        “electromagnetic” field may be electrostatic or magnetic in        nature, or may involve a hybrid of electrical and magnetic        aspects.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made to the embodiments described herein withoutdeparting from the spirit and scope of the invention as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

1. A method of investigating a sample using a charged-particlemicroscope, comprising the steps of: Providing a charged-particlemicroscope, having a particle-optical column; Using the particle-opticalcolumn to direct an imaging beam of charged particles at the sample;Irradiating the sample with the imaging beam, as a result of which aflux of output radiation is caused to emanate from the sample; Examiningat least a portion of said output radiation using a detector,characterized by: Embodying said detector to comprise a Solid StatePhoto-Multiplier that is connected to a power supply providing anadjustable electrical bias; Adjusting said bias so as to adjust a gainvalue of the Solid State Photo-Multiplier; Matching said gain value tothe magnitude of said flux, so as to cause the Solid StatePhoto-Multiplier to operate below its saturation threshold.
 2. A methodaccording to claim 1, wherein: Said portion of the output radiationcomprises particulate radiation; A scintillator is employed to convertat least some of said particulate radiation into photonic radiation;Said photonic radiation is directed to said Solid StatePhoto-Multiplier.
 3. A method according to claim 2, wherein: Thescintillator and Solid State Photo-Multiplier are sandwiched in astacked structure, with an interposed optically transparent separatorlayer; The stacked structure is partially encapsulated in a jacket ofelectrically insulating material, leaving at least a portion of thescintillator exposed.
 4. A method according to claim 1, wherein thedetector is a spatially distributed structure comprising a plurality ofSolid State Photo-Multipliers disposed about a point of intersection ofthe imaging beam and the sample.
 5. A method according to claim 1,wherein the detector is located within the particle-optical column.
 6. Amethod according to claim 5, wherein the sample holder is embodied so asto position the sample within an electromagnetic field of theparticle-optical column.
 7. A method according to claim 1, wherein saidcharged-particle microscope is selected from the group comprising ascanning electron microscope, a transmission electron microscope, ascanning transmission electron microscope, a focused ion beam tool, anelectron-beam-induced deposition tool, an ion-beam-induced depositiontool, a dual-beam charged-particle microscope, a critical dimensionmicroscope, a lithography tool, and hybrids hereof.
 8. A methodaccording to claim 1, wherein said detector is selected from the groupcomprising: A Solid-State Photo-Multiplier; An on-chip pixelated arrayof avalanche photodiodes with shared detection circuitry; A Multi-PixelPhoton Counter, and combinations hereof.
 9. A charged-particlemicroscope constructed and arranged to perform a method as claimed inclaim
 1. 10. A method of investigating a sample using a charged-particlemicroscope, comprising the steps of: irradiating the sample with acharged particle imaging beam causing a flux of output radiation toemanate from the sample; examining at least a portion of said outputradiation using a detector, said detector comprising a Solid StatePhoto-Multiplier that is connected to a power supply providing anadjustable electrical bias; adjusting said bias so as to adjust a gainvalue of the Solid State Photo-Multiplier; and matching said gain valueto the magnitude of said flux, so as to cause the Solid StatePhoto-Multiplier to operate below its saturation threshold.
 11. Themethod according to claim 10, wherein: said portion of the outputradiation comprises particulate radiation; a scintillator is employed toconvert at least some of said particulate radiation into photonicradiation; and said photonic radiation is directed to said Solid StatePhoto-Multiplier.
 12. The method according to claim 11, wherein: thescintillator and Solid State Photo-Multiplier are sandwiched in astacked structure, with an interposed optically transparent separatorlayer; and the stacked structure is partially encapsulated in a jacketof electrically insulating material, leaving at least a portion of thescintillator exposed.
 13. The method according to claim 10, wherein thedetector comprises a spatially distributed structure comprising aplurality of Solid State Photo-Multipliers disposed about a point ofintersection of the imaging beam and the sample.
 14. The methodaccording to claim 10, wherein the detector is located within theparticle-optical column.
 15. The method according to claim 11, whereinthe detector is located within the particle-optical column.
 16. Themethod according to claim 12, wherein the detector is located within theparticle-optical column.
 17. The method according to claim 14, whereinthe sample holder is positions the sample within an electromagneticfield of the particle-optical column.
 18. The method according to claim10, wherein said charged-particle microscope is selected from the groupcomprising a scanning electron microscope, a transmission electronmicroscope, a scanning transmission electron microscope, a focused ionbeam tool, an electron-beam-induced deposition tool, an ion-beam-induceddeposition tool, a dual-beam charged-particle microscope, a criticaldimension microscope, a lithography tool, and hybrids hereof.
 19. Themethod according to claim 10, wherein said detector is selected from thegroup comprising: a Solid-State Photo-Multiplier; an on-chip pixelatedarray of avalanche photodiodes with shared detection circuitry; aMulti-Pixel Photon Counter, and combinations hereof.
 20. Acharged-particle microscope constructed and arranged to perform themethod of: irradiating the sample with a charged particle imaging beamcausing a flux of output radiation to emanate from the sample; examiningat least a portion of said output radiation using a detector, saiddetector comprising a Solid State Photo-Multiplier that is connected toa power supply providing an adjustable electrical bias; adjusting saidbias so as to adjust a gain value of the Solid State Photo-Multiplier;and matching said gain value to the magnitude of said flux, so as tocause the Solid State Photo-Multiplier to operate below its saturationthreshold.